The present invention relates, in general, to a method of growing Group III-V compound semiconductor single crystals of GaAs, InP, GaP, InAs, etc., and Group II-VI compound semiconductor single crystals of ZnSe, ZnS, CdSe, etc., using a liquid encapsulation drawing-up process (LEC process), and an apparatus therefor. The present invention relates, in particular, to an improved method of drawing-up a single crystal using a specific double-crucible apparatus for the purpose of uniformly doping impurities in the single crystal, and an apparatus therefor.
A method of drawing-up a single crystal using a double-crucible apparatus (hereinafter, referred to as a double-crucible method) is used for uniformly doping impurities in the single crystal. If the coefficient of impurity segregation in a molten raw material is not 1, the concentration of impurity in the single crystal will vary as the single crystal is drawn up when the single crystal is grown using a single-crucible method. In order to prevent this variation, a double-crucible method is used.
A double-crucible apparatus comprises an outer and an inner crucible. There are a variety of inner crucibles having various shapes. Double-crucible methods, such as that described above, may be classified into various categories depending on the shape of the inner crucible. A thin through hole is bored in the inner crucible so that molten raw material in the outer crucible is allowed to flow into the inner crucible through the thin through hole. The molten raw material, however, barely passes through the through hole from the inner crucible into the outer one. Therefore, it is possible to keep molten raw material within the inner crucible (hereinafter, referred to as inner molten raw material) and molten raw material within the outer crucible (hereinafter, referred to as outer molten raw material). Inner and outer molten raw material differ in impurity concentration.
Generally, in double-crucible apparatuses, the inner crucible is floated in outer molten raw material. Therefore, the quantity of inner molten is constant independent of the quantity of outer molten raw material. Thus, it is possible to draw up a single crystal having constant impurity concentration if the impurity concentration in the inner and an outer molten raw material are set to be c/k and c, respectively, where k represents a coefficient of segregation of impurity relative to molten impurity components and c represents the desired impurity concentration in the single crystal. In practice, the concentration of impurity in the inner and outer molten raw materials cannot always be set to be c/k and c, respectively, and the coefficient of impurity segregation k is not always equal to a balanced one. Therefore, in practice, it may be impossible to draw up a single crystal having an absolutely constant impurity concentration.
In order to eliminate the foregoing disadvantage, a specific double-crucible apparatus comprising an outer crucible and an inner crucible attached thereto has been proposed. Such a specific double-crucible apparatus as described above is disclosed, for example in U.S. Pat. No. 4,456,499 the substance of which is incorporated herein by reference, as if fully set forth. FIG. 5 is a cross section showing the double-crucible apparatus disclosed in that patent. FIG. 6 shows the impurity concentration when Sb is doped into a single crystal of Si. In the drawing, open and closed circles represent the results obtained using a single-crucible apparatus and the specific double-crucible apparatus, respectively.
In FIG. 5, inner crucible 51, comprising a cylindrical side wall 58 and an expanded annular portion 54, is formed in outer crucible 50. There is communication between the inside of inner crucible 51 and outer crucible 50 through thin through holes 52. Outer crucible 50 is a space defined by outer peripheral wall 53, cylindrical side wall 58, and expanded annular portion 54. The surface level of liquid in outer molten material 57 is always coincident with that of inner molten raw material 56. When single crystal 61 is drawn up out of inner molten raw material 56, inner molten raw material 56 decreases, however, it is compensated to a certain extent because outer molten material 57 flows into inner crucible 51 through thin through holes 52. Inner crucible 51 is not floated in outer molten material 57 and, therefore, the quantity of inner molten material 56 is not constant.
It is possible to draw up a single crystal having constant impurity concentration if respective impurity concentrations c.sub.a and c.sub.b in inner and outer crucibles 51 and 50, respectively, are expressed by the following equations (1) and (2): ##EQU1## where, A and B represent cross sections of inner and outer crucibles 51 and 50, respectively.
As can be seen in FIG. 6, the increase in concentration of impurity is suppressed using this specific double-crucible method. Further, it is described in the prior art reference that the yield of single crystals is improved by 25-30%.
When the liquid encapsulation process is applied to the double-crucible method, however, the problems described hereunder, illustrated in FIGS. 7 to 9, are encountered.
FIG. 7 shows the step in which crystal seeding is carried out. The arrangement of outer crucible 1, inner crucible 2, and thin through holes 3 is the same as shown for the apparatus of FIG. 5. The surface level of an outer molten raw material 4 is coincident with that of an inner molten raw material 5. Further, the surface levels of outer and inner liquid capsules 6 and 7, respectively, are also coincident.
FIG. 8 shows the step in which a shoulder portion of single crystal 10 is formed. In this step, the thickness of liquid capsule 7 in inner crucible 2 increases because the diameter of single crystal 10 is much larger than that of seed crystal 8. When liquid capsule 7 becomes thicker in inner crucible 2, the surface level of inner molten raw material 5 in inner crucible 2 is lowered.
FIG. 9 shows the step in which a straight body portion of single crystal 10 is grown. The space between single crystal 10 and the surface of the wall of inner crucible 2 becomes smaller, and, therefore, the thickness of liquid capsule 7 becomes larger. As a result, the difference between inner surface 11 and outer surface 12 of inner and outer molten raw materials, respectively, increases further.
The temperature of the molten raw materials is higher than the melting point of the same, while the temperature of single crystal 10 is lower than the melting point thereof. The temperature at inner surface 11 is, of course, equal to the melting point of single crystal 10. However, the temperature at peripheral edge Q of the inner surface 11 is higher than the melting point because the temperature is influenced by the temperature of outer molten raw material 4. Further, the temperature at inner end point P on a top surface of outer molten raw material 4 is higher than the melting point of single crystal 10. Thus, a steep temperature gradient in the radial direction has to exist between the outer peripheral surface of single crystal 10 in inner crucible 2 and the inner peripheral wall of inner crucible 2.
Generally, in the method of drawing-up a single crystal, a vertical temperature gradient can be formed and easily controlled. However, it is difficult to control a temperature gradient in the radial direction. It is difficult to attain even formation of a temperature gradient in the radial direction in the vicinity of center portion of the crucible, and it is also difficult to keep the temperature gradient in the radial direction constant.
In the state shown in FIG. 9, there is a risk of remelting single crystal 10 from the side periphery thereof due to heat given thereto. Therefore, while short single crystals can be formed, it is impossible to produce elongated single crystals. In addition, single crystals may also be uneven in diameter and may have defects. In the extreme case, no single crystal may be grown as a result of a unstable temperature gradient in the radial direction.
Thus, in the method of drawing-up a single crystal, it is generally not desirable to form a temperature gradient in the radial direction. To this end, a difference in the surface level between inner and outer molten raw materials must not be allowed to exist.